1. Field of the Invention
This invention relates to a signal communication system, and more particularly to an improved signal communication system which conveys signals between power boards, control boards or other board devices (hereinafter referred to merely as "board devices") with a minimum of wires in a signal communication cable.
2. Description of the Prior Art
A typical example of conventional wiring between board devices of the above mentioned sort is illustrated in FIG. 1. In FIG. 1, there are shown two board devices generally designated the reference numbers 100 and 200. The two board devices 100 and 200 are connected through a communication cable 300. The board devices 100 and 200 each includes a terminal board 150 or 250. The communication cable 300 is connected between the terminal boards 150 and 250. The terminal boards 150 and 250 have terminals P, N, 1, 2 ... n wherein P and N are terminals for power supply. Power supply lines 400 include a positive line P and a negative line N. There are also shown relay coils R.sub.11, R.sub.12 and R.sub.21 and their corresponding relay contacts C.sub.11, C.sub.12, C.sub.21 and C.sub.22. In the example of FIG. 1, power voltages P and N are fed from one of the board devices to the other through the cable 300.
The system as illustrated in FIG. 1 will operate as follows. Upon closure of the contact C.sub.11 the coil R.sub.21 is energized and upon closure of the contact C.sub.21 the coil R.sub.11 is energized. Furthermore, upon closure of the two contacts C.sub.12 and C.sub.22 the coil R.sub.12 is placed into its energized state. In other words, the two board devices 100 and 200 are rendered operative in association with each other through the terminal boards 150 and 250.
The problems of the system as shown in FIG. 1 are that it requires a number of core wires in the connection cable 300 and becomes expensive correspondingly with increase in the distance between the two devices 100 and 200. One solution to those problems is a system of FIG. 2 by which to reduce the number of connection wires to a minimum.
FIG. 2 schematically illustrates an improved prior art wiring scheme between the board devices for further simplicity of connections between the devices as shown in FIG. 1 and minimization of the signal communication cable. In FIG. 2, components similar to those in FIG. 1 are represented by the same reference numbers. The board devices 100 and 200 include conventional signal communicators 10 and 20 for such wiring. The signal communicators 10 and 20 each are divided into a transmitter side and a receiver side. On the transmitter side of the signal communicator 10 signals are conveyed to a cable 310 in the signal communication cable 300 by way of an input signal terminal board 101, a signal converter circuit 102, a parallel-to-serial conversion logic circuit 103, a modulator 104 and a transmitter/receiver terminal board 120. The signals from the cable 310 are received through a transmitter/receiver terminal board 220, a demodulator 214, a serial-to-parallel conversion logic circuit 213, an output conversion circuit 212 and an output signal terminal board 211 on the receiver side of the signal communicator 20. In a likewise manner, signals are conveyed to a cable 320 in the signal communication cable 300 by way of an input signal terminal board 201, a signal conversion circuit 202, a parallel-to-serial conversion logic circuit 203, a modulator 204 and transmitter/receiver terminals 220 on the transmitter side of the signal communicator 20. The signals from the cable 320 are received through the transmitter/receiver terminal board 120, a demodulator 114, a serial-to-parallel conversion logic circuit 113, an output conversion circuit 112 and an output signal terminal board 111 on the receiver side of the signal communicator 10. The board devices 100 and 200 include individual power supply lines 401 and 402. The signal communicators 10 and 20, respectively, include power supply conversion circuits 131 and 231 which are led to power supply lines 401 and 402 via power terminal boards 130 and 230, for the purposes of generating voltages for logic operations.
Operation of the above described system will now be discussed. The input signal terminal board 101 in the board device 100 of FIG. 1 is fed with a positive power supply voltage P.sub.1, depending upon whether or not the contacts C.sub.11, C.sub.12 and so forth are closed. The respective terminals of the input signal terminal board 101 are connected to respective input terminals of the signal conversion circuit 102. The signal conversion circuit 102, when its respective input terminals are connected to the power supply P.sub.1 and supplied with DC 110V, for example, provides a logic "1" voltage (typically, DC 5V) at its output terminals corresponding to the respective input terminals. For those input terminals that are not connected to the power supply P.sub.1 and remain open, the signal conversion circuit provides a logic "0" voltage (typically, DC 0V) at its output terminals corresponding to those input terminals. The logic signals at the respective output terminals of the signal conversion circuit 102 are fed to respective parallel input terminals of the parallel-to-serial conversion logic circuit 103 which in turn converts those signals into bit serial signals which are supplied to the modulator 104 after error detection codes such as parity codes have been added thereto. The modulator 104 modulates a carrier suitable for transmission with the input signals and sends out the same to the signals communication cable 300 via the transmitter/receiver terminal board 120. For example, the carrier is within an audio frequency range and frequency-modulated with the input signals. Typically, 2100 Hz sinewave signals are provided for the logic "0" signals and 1300 Hz sinewave signals for the logic "1" signals. This permits transmission to take place under the condition where signals being transmitted are immune to noise readily induced over the cable.
The modulated carrier conveyed over the signal communication cable 300 are then demodulated through the demodulator 214 into bit serial signals which in turn are converted through the serial-to-parallel conversion circuit 213 into parallel signals and fed to the output conversion circuit 212. Within the output conversion circuit 212, a voltage P.sub.2 is applied to terminals of the serial-to-parallel conversion logic circuit 213 which correspond to logic "1" bits whereas terminals thereof which correspond to logic "0" bits are held in open state. The outputs of the output conversion circuit 212 are connected to the relay coils and contacts through the output signal terminal board 211. Signal transmission from the transmitter side of the signal communicator 20 to the receiver side of the signal communicator 10 takes place in the same manner as with signal transmission from the transmitter side of the signal communicator 10 to the receiver side of the signal communicator 20.
Therefore, should the contact C.sub.11 be brought into a closed position, the power supply voltage P.sub.2 is fed to the terminal 1 of the output signal terminal board 211, thus energizing the coil R.sub.21. Upon closure of the contact C.sub.21 the power supply voltage P.sub.1 is supplied to the terminal 1 of the output signal terminal board 111 to energize R.sub.11. Upon closure of the contact C.sub.12 the power supply voltage P.sub.2 is supplied to the terminal 2 of the output signal terminal board 211. If under this circumstance the contact C.sub.22 is placed into a closed position, then the power supply voltage P.sub.2 is fed to the terminal 2 of the input signal terminal board 201, permitting the supplying of the power supply votage P.sub.1 to the terminal 2 of the output signal terminal board 111 and the energizing of the coil R.sub.12.
With the above described system, it is possible to convey a number of signals through a minimum of communication lines (the lines 310 and 320 in the example of FIG. 2). However, in the device of FIG. 2, there are two kinds of terminal boards, the input signal terminal board and the output signal terminal board, which are separated from each other. The designer of the board devices is required to consider arrangement of various terminals, components and the like in the board deivces while taking into consideration the location of the terminal boards of the signal communicators settled in the board devices. In other words, a limitation is imposed that terminal arrangement in the board devices should be determined in association with the location of the signal communicators, presenting great difficulties in the design of the board devices. Another problem is that great difficulties are experienced in applying the system of FIG. 2 to the board devices designed without consideration of the foregoing limitation.